The present invention relates to a ratchet mechanism through which a rotational torque is transmitted in one direction, in which at least one of members contacting against each other is made of synthetic resin.
It is advantageous to use synthetic resin to make ratchet mechanisms used in compact devices such as a timer and a music box to which a relatively small mechanical load is applied, in view of a reduction of the mechanical parts to be used, a facilitation of assembly and a reduction of machining cost. Therefore, there have been various attempts to provided ratchet mechanisms having rotational members and transmission members made of synthetic resin.
For example, the Japanese Utility Model Publication No. 55-34342 discloses a ratchet mechanism attaining the above noted objects, but the proposed ratchet mechanism still has various defects. Namely, if the condition in which an engagement distal end of a ratchet arm is kept to be in contact with a tooth of an associated ratchet gear, the arm is forcibly moved outwardly, so that a creep phenomenon would occured in the arm. As a result, the engagement between the distal end of the arm and the tooth of the gear is insufficient, and the distal end of the arm or the tooth would be damaged.
In addition, in the case that the ratchet arm is formed integrally with an outer gear, it is necessary to impart a predetermined amount of movement or flexibility to the engagement portions. For this reason, to prevent a deformation and a damage of the arm, it is necessary to elongate the arm. This leads to the limitation of the number of the arms. For example, in the case that the spring is repeatedly operated, the returning angle of the windup shaft immediately after completion of winding is preferably small for facilitating the winding operation. However, the operational characteristics would deteriorate with a small number of the arms.
Also, with only one engagement portion, upon rotational transmission of torque, the load is applied to the one engagement portion, so that the driven and/or driving members are eccentrically deformed.
To overcome this defect, the Japanese Utility Model Publication No. 56-17871 discloses a ratchet mechanism having a plurality of arms for engagement. However, in that ratchet mechanism, the engaging tooth is one. For this reason, the load would be applied eccentrically thereto upon torque transmission.
Also, a distance between the transmission member formed on the rotational member made of synthetic resin and the engagement portion engaging with the transmission member must be increased in view of the mechanical strength. In addition, also in view of the mechanical strength, the size of each member must be increased. Therefore, in the ratchet mechanism having parts made of synthetic resin, it is difficult to reduce the rotational member in thickness and in diameter and it is impossible to provide a number of tooth steps.